The present invention relates to the production of microcapsules containing as the core material oily substances or water-insoluble solid particles. More particularly, the present invention relates to a process for producing microcapsules which comprises forming a wall of coacervates on microdroplets of an oily substance of water-insoluble solid particles through reaction between gelatin and an anionic polymer, and allowing an iridoid compound to act on the coacervates so that cross-linking occurs between the polymer molecules of gelatin to harden the microcapsules.
Microcapsules which are tiny particles of a core material surrounded by a coating consisting of a wall material such as gelatin serve to protect the core material from its surroundings by means of the wall membrane, or to control the time, place or rate at which the core material is released. Having these capabilities, microcapsules have been extensively used in pressure-sensitive copy paper, foodstuffs, pharmaceuticals and in many other fields of industry.
One of the common methods for producing microcapsules is "complex coacervation" which is commercially the most common for pressure-sensitive copy paper. In this process, two colloidal substances, such as gelatin and an anionic polymeric substance, having mutually opposite electric charges, are added to a core-containing suspension to form an aqueous sol, which is then pH-adjusted or otherwise treated to form a wall of coacervates on the microdroplets of an oily core material, and after they have gelled the coacervates are hardened with a hardening agent to form microcapsules. Details of this complex coacervation process are disclosed in U.S. Pat. No. 2,800,457, etc. and many methods of improvement have been proposed.
For instance, Japanese Patent Publication No. 39-24782 proposes that the temperature of the system be slowly elevated in the presence of a hardening reagent in order to shorten the duration of the hardening treatment. Japanese Patent Publication Nos. 47-16166, 47-16167 and 47-16168 propose that in order to prevent agglomeration of coacervates, an anti-shock agent be added after the wall membrane of coacervates has been gelled. Japanese Patent Publication Nos. 50-27827, 50-27828 and 50-27829 show that by adding an anti-shock agent such as CMC or an acrylic acid copolymer together with an anionic surfactant, it is possible to prevent any increase in viscosity that would otherwise occur on account of the reaction taking place between gelatin and aldehyde during the pre-hardening step. Japanese Patent Public Disclosure No. 61-4527 discloses a method in which a water-soluble wax derivative is added after gelation and before hardening. Methods employing hardening agents that may be added to foods are disclosed in Japanese Patent Public Disclosure Nos. 59-36540, 60-37934, 61-4527, 61-78351, etc.
As described above, numerous improvements have been developed in the technology of microencapsulation by complex coacervation. However, there still remains much room for improvement in the art of hardening coacervates. For example, no process has yet been developed for producing microcapsules employing a hardening agent which can be safely incorporated in foodstuffs, which is stable, and which allows the hardening treatment to be effected within a short period. Hardening agents that are most commonly employed in the conventional techniques of complex coacervation are aldehydes such as formaldehyde and glutaraldehyde. However, aldehydes are toxic and cannot be used in the production of microcapsules to be incorporated in foodstuffs.
A further problem with the use of aldehydes as hardeners is that the pH of the system has to be adjusted to the alkali side in order to increase the rate of hardening reaction. Unless utmost care is exercised in the pH adjustment, agglomeration of coacervates and other troubles will occur to increase the complexity of process control.
Hardeners that have been employed in the production of microcapsules for incorporation in foodstuffs include glucono delta lactone, tannic acid, potassium alum [KAl (SO.sub.4).sub.2.12H.sub.2 O] and ammonium alum [NH.sub.4 Al (SO.sub.4).sub.2.12H.sub.2 O]. Glucono delta lactone and tannic acid harden gelatin by causing acid denaturation of the gelatin protein, and the alum compounds achieve the same result by binding between the molecules of gelatin protein. However, all of these hardeners are slow in their hardening rate because their action is weaker than that of aldehydes. If the reaction temperature is raised in an attempt to increase the hardening rate, the chance of coacervates being broken is increased. Under these circumstances, the hardening reaction must be carried out over a prolonged period (usually several tens of hours) at room temperature. Furthermore, the wall membrane of the resulting capsules is weak and has insufficient heat resistance.